Keep Track
You can catch the last departure and compare migrations year-to-year if you keep track of your data. Here's how one observer determined her last adult male hummingbird of the season:

"Our final sighting of our adult male hummer was last Thursday morning (August 29) at 6:00 am. It's now September 2nd, so it's the 4th day we haven't seen him. His presence was always obvious so I think it's safe to say he's gone! Beginning August 16th, we kept a tally and recorded each day we saw him. It was a fun way to keep track and catch his departure."